Luminosity (8)
I believe the answer is:
radiance
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">luminosity</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for radiance that I've seen before include "A bright glow (E, not Y)" , "shine" , "This light" , "Brightness" , "Glow of joy" .)